An image sensor is a device that can convert an optical image into an electronic signal. Image sensors are oftentimes utilized in still cameras, video cameras, video systems, and other imaging devices. Cameras and other imaging devices commonly employ either a charge-coupled device (CCD) image sensor or a complementary metal-oxide-semiconductor (CMOS) image sensor.
CMOS image sensors include an array of pixels, each of which can comprise a photodetector. CMOS image sensors also include circuitry to convert light energy to an analog voltage. Moreover, CMOS image sensors can include additional circuitry to convert the analog voltage to digital data. Thus, a CMOS image sensor can be an integrated circuit that comprises various analog, digital, mixed-signal, etc. components associated with capturing light and processing imaging related information; accordingly, a CMOS image sensor can be a system on chip (SoC). For example, components integrated into the CMOS image sensor oftentimes include a processor module (e.g., microprocessor, microcontroller, or digital signal processor (DSP) core), memory, analog interfaces (e.g., analog to digital converters, digital to analog converters), and so forth.
Visible imaging systems implemented using CMOS image sensors can reduce costs, power consumption, and noise while improving resolution. For instance, cameras can use CMOS image sensors that efficiently marry low-noise image detection and signal processing with multiple supporting blocks that can provide timing control, clock drivers, reference voltages, analog to digital conversion, digital to analog conversion, key signal processing elements, and the like. High-performance video cameras can thereby be assembled using a single CMOS integrated circuit supported by few components including a lens and a battery, for instance. Accordingly, by leveraging CMOS image sensors, camera size can be decreased and battery life can be increased. Also, dual-use cameras have emerged that can employ CMOS image sensors to alternately produce high-resolution still images or high definition (HD) video.
CMOS image sensors oftentimes include voltage boosters; thus, a voltage booster can be integrated as part of a CMOS image sensor system on chip. The voltage booster can generate a positive boosted output voltage that is above a supply voltage (e.g., above 3.3 V or any other supply voltage) and/or a negative boosted output voltage that is below ground (e.g., below 0 V). The voltage booster can be used to allow for high pixel performance at low supply voltages. According to an example, the voltage booster can generate a positive boosted output voltage, which can be used to drive transfer gates and/or reset gates of pixels in a pixel array of the CMOS image sensor to provide a wide dynamic range. By way of another example, the voltage booster can generate a negative boosted output voltage, which can be provided to transfer gates of pixels in the pixel array of the CMOS image sensor to reduce dark current.
Load capacitance on a voltage booster can vary considerably as a function of operating conditions. A voltage booster is typically designed to handle maximum loading with associated disturbance loads. However, when loading is less than the maximum loading, an output voltage (e.g., positive boosted output voltage or negative boosted output voltage) can include excessive ripple. Waveforms of output voltages that include excessive ripple can overshoot target voltage ranges for the output voltages, which can detrimentally impact performance of a CMOS image sensor.
A conventional approach for mitigating ripple in an output voltage waveform of a voltage booster includes employing a linear regulator in the voltage booster. However, linear regulators typically require power and occupy significant area on a chip. Yet, chips of common CMOS image sensors oftentimes lack sufficient area for linear regulators. Another common approach to reduce ripple in the output voltage waveform is to add ballast capacitance to the voltage booster. The ballast capacitance can increase a capacitance between an output node of the voltage booster and ground, which can reduce ripple under lower loading conditions. However, ballast capacitors can also occupy significant area on a chip, which may be unavailable.